This invention relates to the coating of nickel with gold as, for example, is required in the preparation of ceramic carriers employed in the packaging of semiconductor chips.
The increased performance and circuit/bit densities of today's semiconductor chips require corresponding technological advancements in chip packaging. Since the introduction of the leadless ceramic chip carrier, the chip carrier packaging concept has seen increasing use. Ceramic chip carriers typically make use of alumina-based substrates and have discrete areas of multi-layer metallization bonded to the ceramic substrate. These areas of metallization comprise in sequence (a) a base metallization layer bonded to the ceramic substrate, (b) a layer of nickel bonded to the initial, or base, layer and (c) a layer of gold bonded to the nickel layer.
In the typical fabrication of multi-layer ceramic substrates alumina powder is mixed with glass frit and organic chemicals to form a slurry. This slurry is cast into sheets having a controlled thickness, which sheets are then blanked into various sizes and shapes, and via holes are punched. These green sheets are then screen printed with tungsten (or molybdenum) to form the base metallization. These metallized green sheets are stacked, and laminated together, followed by cofiring (i.e., sintering) in hydrogen or a hydrogen-nitrogen mixture with the heating schedule usually peaking at 1550.degree. C. -1650.degree. C. Thereafter, these sintered substrates are processed to apply nickel metallization over the exposed discrete areas of sintered tungsten. This is followed in turn by gold metallization of the nickel surfaces. Actual compositions of the slurry and specifics of the processing can be expected to vary from manufacturer to manufacturer.
The tungsten metallization is about 10 micrometers thick and is very porous. The nickel layer applied thereto is typically 2-5 micrometers thick and applied by either electrolytic or electroless nickel plating. The nickel functions both to aid in wire bonding and to provide a better thermal expansion match between the tungsten and gold layers on opposite sides thereof. The thin layer of gold, typically 1-2 micrometers thick, is applied to accommodate die attachment, wire bonding and sealing. For good hermeticity (and for other reasons) it is important that the nickel and the gold metallizations contain as few pores as possible.
In U.S. patent application Ser. No. 740,377--Park, "Nickel Coating Diffusion Bonded To Metallized Ceramic Body And Coating Method", filed June 3, 1985, now U.S. Pat. No. 4,590,095 pack cementation is used to diffusion bond substantially pore-free nickel to discrete exposed areas of tungsten or molybdenum that are in turn bonded to the surface of a ceramic body. The preferred method for using the gold plating formulation of this invention is described in U.S. patent application Ser. No. 753,094--Zarnoch and Iacovangelo, "Control of Cyanide Activity In Electroless Plating Compositions", filed July 9, 1985. Both the aforementioned applications are assigned to the assignee of the invention described and claimed herein and are incorporated by reference.
The sintered multi-layer ceramic bodies provided with the discrete areas of multi-layer metallization are subsequently subjected to brazing, chip joining and capping operations.
High purity nickel and gold deposits generally can be obtained by electrolytic plating. It is well known, however, that this process has several major drawbacks including the following: (a) because of the need for an externally applied electrical current, it is often difficult to plate articles with complex shapes and circuitry; (b) for the same reason, the resulting nickel or gold coating is generally very nonuniform being thicker in well-exposed areas and substantially thinner at corners, and (c) such coatings tend to be porous.
Because of these and other disadvantages of the electrolytic mode of plating, nickel and gold metallizations of ceramic chip carriers increasingly are carried out by electroless plating. This latter method can plate articles regardless of complexity of shape or circuitry with a relatively uniform coating thickness. This result is due to the fact that the surface to be plated is subjected to pretreatment rendering the surface catalytic to the metal deposition and each unit area should be equally catalytic. Additionally, metals deposited electrolessly plate well into holes and around corners.
U.S. Pat. No. 3,700,469--Okinaka discloses an autocatalytic (i.e., electroless) gold bath formulation employing a gold cyanide complex, excess free cyanide, a pH adjusting agent and a borohydride or amine borane as the reducing agent. Such a bath exhibits advantages over displacement electroless baths, since the latter dissolve the substrate material and are also incapable of allowing higher plating thicknesses. However, the composition described in Okinaka has its own limitations: inadequate stability, low gold plating rate, difficulty in replenishing the bath, and sensitivity to the presence of nickel ions.
The electroless gold bath composition disclosed in U.S. Pat. No. 3,917,885--Baker employing an aqueous solution of an alkali metal imide complex of the metal to be plated purportedly exhibits improved stability, but is found to suffer from the same problems as Okinaka, especially sensitivity to nickel contamination.
U.S. Pat. No. 4,337,091--El-Shazly et al. proposes the use of trivalent gold metal complexes as the source of gold in an electroless gold plating bath, the reducing agent being any of the borohydrides, cyanoborohydrides or amine boranes that are soluble and stable in aqueous solution. A later version of the El-Shazly et al. electroless gold plating bath using a mixture of trivalent and monovalent water-soluble gold components is described in U.K. Patent Application G.B. No. 2121444A. This latter item of prior art suggests the use of hydrazine as a reducing agent and speaks of plating rates of up to 8 microns per hour. As with the other prior art electroless gold plating formulations the El-Shazly baths are easily contaminated by nickel.